High intensity discharge (HID) lamps include high pressure sodium lamps, metal halide lamps and mercury vapour lamps. They are distinct from for example fluorescent lamps. For example, they tend to operate under higher pressures and temperatures, they emit visible light directly (although they may also include some phosphor coatings), and they are generally compact light sources having arc tubes with small electrode spacings. They tend to have relatively high power ratings, for example above 150 Watts.
Gas discharge lamps in general require some form of regulation element to allow for their start-up and operation. There are a number of different controllers on the market for fluorescent lamps, but less so for HID lamps.
Generally, a HID lamp system will use an inductive ballast that is designed to make the lamp operate at its approximate design power assuming a constant supply voltage. In practical installations, however, the input supply voltage can change due to supply line disturbances and the like, and this can result in a lamp being over-driven or under-driven. Other factors can also affect the proper running of a lamp, such as changes caused by lamp aging, by differences between lamps due to manufacturing tolerances, and by environmental conditions.
If a lamp is over-driven, it can exhibit adverse characteristics such as overheating, reduction in lamp life, electrode damage, reduced efficiency and low colour temperature. Conversely, if a lamp is under-driven, adverse characteristics of low light output, reduced efficiency, high colour temperature and arc tube blackening tend to result.
An aim of the present invention is to provide new and advantageous controllers, circuits and methods for controlling the operation of HID lamps.